The influence of micro- and nano- sisal fibres on the morphology and properties of different polymers
Ahmad, Essa Esmail Mohammad
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In this study, three types of polyethylene, low-density (LDPE), linear low-density (LLDPE), and high-density (HDPE) polyethylene, were used as polymer matrices to prepare sisal fibre reinforced polyethylene composites containing 10-30 wt% fibre. The untreated and the dicumyl peroxide (DCP) treated composites were prepared by melt mixing, followed by hot melt pressing. The influence of the DCP treatment, the polyethylene molecular characteristics, and the sisal fibre loadings on the morphology and on the thermal, mechanical, and dynamic mechanical properties of the composites was investigated. The gel contents of the composites varied significantly depending on the polyethylene molecular characteristics. The LLDPE composites had the highest gel content values followed by LDPE and then HDPE, for which the gel content did not change significantly. These results strongly suggested the presence of grafting of the polyethylene chains onto the sisal fibre surfaces combined with crosslinking between the polymer chains. The morphologies of the cryofractured surfaces and the xylene-extracted samples further confirmed the presence of the grafting, particularly in the case of the treated LLDPE and LDPE composites. The SEM micrographs of the treated LLDPE and LDPE composites showed better interfacial adhesion between the polymers and the sisal fibres. For HDPE composites, however, such interfacial bonding was not observed from the SEM micrographs. The SEM images of all the untreated polyethylene composites showed poor interfacial interactions. TGA analyses showed that the treatment did significantly affect the thermal stabilities of the composites, and all the untreated and the treated samples were thermally less stable than the neat polymer matrices. The DSC results demonstrated that the crystallization and melting behaviour of all the untreated polyethylene composites remained unaffected. However, both the DCP treatment and the sisal fibre loadings to some extent influenced the crystallization and melting behaviour of the LLDPE composites, whereas those of the LDPE composites were only slightly affected. The treated HDPE composites, however, did not show significant changes in their crystallization and melting behaviour. The elongation at break for all the treated and the untreated polyethylene composites showed similar trends and the treatment did not bring about any differences. Compared to the untreated composites, the tensile strength and the Young’s modulus of the treated LLDPE and LDPE composites were remarkably higher, whereas the Young’s modulus of the treated HDPE composites was observably lower and no significant effect on the tensile strength was noticed. The storage modulus of the LLDPE and LDPE composites showed good correlation with the tensile testing results. The tan δ curves showed a slight increase in the glass transition temperatures for the treated composites. The storage modulus of the treated HDPE composites remarkably decreased, and the tan δ curves did not show the β-relaxation as in the case of the other two polymers. The effect of the incorporation of sisal whickers on the properties of poly(lactic acid) was also investigated in this study. Untreated and the MA/DCP and DCP treated PLA nanocomposites, with sisal whiskers loadings of 2 and 6 wt%, were prepared by melt mixing and hot melt pressing. The dispersion of the whiskers in the PLA matrix as well as the thermal and viscoelastic properties of the nanocomposites were determined using TEM, DSC, TGA, and DMA. The dispersion of the whiskers was found to be similar, whether the samples were treated or not. The presence and the amount of whiskers in the untreated nanocomposites slightly decreased the calculated percent crystallinity, but the Tm, Tc and Tg remained fairly constant compared to neat PLA. The type of treatment was also found to influence the crystallization and melting behaviour of the nanocomposites. The TGA results showed that neither the sisal whiskers loading nor the treatment had a significant effect on the thermal stabilities of the nanocomposites. The incorporation of the whiskers remarkably reduced the intensity of the glass transition in the tan δ curve, and all the nanocomposites showed higher storage modulus values compared to the neat PLA. The type of treatment did not really influence the stiffness of the samples. Entirely bio-based nanocomposites of PFA and sisal whiskers were prepared by an in situ polymerization method. The effect of increased sisal whiskers loadings (1 and 2 wt%) on the thermal and the dynamic mechanical properties of the nanocomposites were studied. No significant changes in the thermal stabilities of the nanocomposites could be seen. The storage moduli of the nanocomposites were significantly increased by the presence and the amount of sisal whiskers, and the intensity of the glass transition relaxation in the tan δ curve observably decreased and slightly shifted to lower temperatures.